This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-380321, filed Dec. 13, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a magnetic memory device and a manufacturing method thereof, and more particularly, to a magnetic memory device and a manufacturing method thereof for writing data by use of a current magnetic field in each bit, and reading out information of xe2x80x9c1xe2x80x9d, xe2x80x9c0xe2x80x9d according to the resistance change caused by the change in the cell magnetization state.
2. Description of the Related Art
Recently, an MRAM (Magnetic Random Access Memory) utilizing a tunneling magneto resistive (TMR) effect is proposed as a memory element. MRAMs have been developed to have non-volatility, high-density integration, high reliability and high operation speed thus have great potential in the memory market.
FIG. 26 is a cross sectional view showing part of an MRAM according to the prior art technique. As shown in FIG. 26, first and second wirings 13, 23 are arranged to cross at right angles and an MTJ (Magnetic Tunneling Junction) element 16 is arranged in the cross point portion between the first wiring 13 and the second wiring 23. The MTJ element 16 is connected to the second wiring 23 via an upper electrode (not shown) and connected to a source/drain diffusion layer 52 of a MOS transistor 53 via a lower electrode 55 and contact 54. Further, a gate electrode 51 of the MOS transistor 53 is used as a readout wiring.
The MTJ element 16 is configured by a magnetically fixed layer 31 which is a ferromagnetic layer and connected to the lower electrode 55, a magnetic recording layer 33 which is a ferromagnetic layer and connected to the second wiring 23 via the upper electrode and a tunnel junction layer 32 which is a non-magnetic layer and sandwiched between the magnetically fixed layer 31 and the magnetic recording layer 33.
In the above MRAM, the data write and readout operations are performed as follows.
First, when data is written into a desired selected cell, the state of xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d data is written into the MTJ element 16 of a selected cell by inverting the magnetization direction of the magnetic recording layer 33. As a result, the resistance of the tunnel junction layer 32 becomes the lowest when the magnetization direction of the magnetic recording layer 33 become the same as the magnetization direction of the magnetically fixed layer 31. In contrast, when the magnetization directions become opposite to each other, the resistance of the tunnel junction layer 32 becomes the highest. A change in the resistance of the tunnel junction layer 32 is read by causing a current to flow in a direction through the MTJ element 16 from the two wirings 23, 13 arranged above and below the MTJ element 16 with the upper electrode and lower electrode 55 which sandwich the MTJ element 16 disposed therebetween. Thus, the storage state of xe2x80x9c1xe2x80x9d, xe2x80x9c0xe2x80x9d can be determined and information can be read out.
As described above, in the MRAM of the prior art technique, two-value data can be stored for each bit, but it is impossible to store data having a larger value.
A magnetic memory device according to a first aspect of the present invention, comprises magneto resistive elements which are laminated in each cell with the easy axes of magnetization (easy axes) set in different directions, each magneto resistive elements having at least two resistance values, and first and second wirings which sandwich the magneto resistive elements and arranged to extend in different directions.
A magnetic memory device manufacturing method according to a second aspect of the present invention, comprises forming a first wiring which extends in a first direction, laminating magneto resistive elements above the first wiring, the magneto resistive elements respectively having magnetically fixed layers and each having two resistance values, forming a second wiring which extends in a second direction different from the first direction on the magneto resistive elements, and sequentially performing heat treatments in a magnetic field at different temperatures with respect to the magneto resistive elements and fixing magnetization directions of the magnetically fixed layers of the magneto resistive elements in different directions.